Glaucoma is a major cause of irreversible blindness in the United States. Approximately one percent of the general population has this disease. Visual loss from glaucoma is usually due to elevated intraocular pressure which damages the optic nerve. The cause for the elevated intraocular pressure is not entirely known, but, at least in many cases, appears to be due to an increase in the resistance to the outflow of aqueous humor from the eye. Aqueous humor is a fluid which circulates within the anterior portions of the eye and provides nourishment to and removes waste products from various avascular structures in the eye such as the cornea and the lens. Aqueous humor is continuously being formed within the ciliary body epithelium and is normally continuously drained out of the eye through the trabecular meshwork and Schlemm's Canal into the venous system. It is believed that the site of resistance to the outflow of aqueous humor is in the juxtacanalicular portion of the trabecular meshwork.
Treatment of glaucoma is, therefore, usually directed at lowering the intraocular pressure and thereby preventing progressive optic nerve damage and visual field loss. If medical therapy (topical eyedrops and oral tablets) and laser therapy fail to adequately lower the intraocular pressure, then glaucoma surgery is required. There are several different types of glaucoma surgery (filtration, goniotomy, trabeculotomy, cyclodialysis, etc.), but all of them have a common purpose: that is, to bypass or eliminate any blockage and thereby restore the outflow of aqueous humor from the eye.
Goniotomy and trabeculotomy surgery involve incising the abnormal tissue which is overlying or inside the trabecular meshwork in order to allow aqueous humor to flow freely into the normal outflow system. These two procedures are most commonly used in treating childhood glaucomas, but also have limited success in adult-onset glaucomas. When these surgical procedures fail, the failure is usually due to scarring which blocks the incision in the trabecular meshwork. In an attempt to avoid this problem, a technique was developed in which an intraocular diathermy probe is used to cauterize the incision edges in an attempt to reduce or eliminate scarring. However, this technique has not been proven to be any more successful than the other surgical procedures.
In identifying the reason for open-angle glaucoma and for failure of these surgical procedures and in determining the extent of the blockage and the preferred treatment, it would be useful to extract relatively large intact samples of undamaged trabecular meshwork and scar tissue and, perhaps, the surrounding tissue for histopathologic examination. Unfortunately, existing instruments for performing this surgery do not simultaneously permit the cutting and removal of relatively large and undamaged tissue specimens. The use of separate instruments as are known in the art for cutting and excision in a delicate operation like glaucoma surgery is not a satisfactory alternative. Such a procedure may entail a plurality of surgical penetrations of the inner eye thereby increasing the dangers of trauma, injury, and infection. In some cases it may also be impossible to locate a severed segment of tissue from the inner eye when using a second instrument to retrieve it. In addition, such a two-step procedure is certain to require more time than would a one-step procedure, a particularly important factor if a plurality of tissue samples are desired.
Those surgical instruments in the prior art which are capable of both cutting and removing tissue specimens in a single operation are, in general, either not adaptable for delicate glaucoma surgery, do not remove large enough tissue samples, or else result in damage to the structure of the excised tissue. For example, the prior art includes a plethora of biopsy instruments which both cut and remove tissue. In testing for carcinomous tissue, however, only relatively small tissue samples are required, and the physical structure of the tissue is usually not important. Therefore, these instruments cannot be adapted for the excision of relatively large, undamaged segments of glaucoma scar tissue. Some of the more pertinent prior art patents in this field are discussed below, and all of these patents are specifically incorporated herein by reference.
U.S. Pat. No. 3,996,935 (Banko II) is perhaps the most representative of the prior art in this field, and of its limitations. Banko II discloses a multi-purpose instrument designed for ophthalmic surgery including removal of foreign matter, blood clots, lenses and other eye tissue. The instrument includes an internal probe having a distal end for engaging tissue or foreign matter to be removed from the interior of the eye. The internal probe comprises two coaxial shafts, each having a jaw or port at its terminal end, such that the inner shaft is slidable and rotatable with respect to the outer. In other embodiments of the Banko II device, the hollow interior of the inner coaxial shaft can be utilized as a fluid irrigation channel or as a suction canal to facilitate tissue removal.
The Banko II patent, however, requires suction to engage and remove tissue. Banko's "internal probe" or "inner jaw" has cutting edges to cut tissue against the sharp opening edges of the "outer jaw" at the end of the outer hollow shaft resulting in a scissor-like cutting action which could damage the tissue. This instrument is designed to cut tissue into small pieces for removal, not to extract a relatively large, undamaged and intact tissue segment. The shape of this instrument also makes it unsuitable for goniectomy surgery. This instrument is also relatively complex, requiring an external power source for applying suction and for continuously rotating the "internal probe." Accordingly, this instrument is not suitable for conducting goniectomy surgery.
U.S. Pat. No. 3,583,390 (Jascalevich) is an example of a typical biopsy instrument. This patent discloses a biopsy device which has generally tapered, two-sided arrowhead-shaped cutting tool with a hole near the tip of the cutting edges. The shape of this biopsy instrument makes it unworkable for goniectomy surgery because the opening that would be required to introduce the instrument into the eye would be too large. Also, the large exposed surface area of the blades could cause trauma or injury to adjacent intraocular tissue during the surgical procedure. With the instrument of this patent, the tissue specimen is cut and separated from adjacent tissue by two blades -- the biopsy blade and the scalpel blade -- operating in a scissor-like fashion. Use of the Jascalevich instrument in the eye would necessitate making three entry wounds, one for the biopsy device and two for the scalpel blade (one for each groove). Because the two blades must be separately manipulated to excise a tissue specimen, unless a high degree of care and skill is exercised by the surgeon severe injury to the surrounding eye tissue could result. Upon withdrawing the biopsy instrument from the eye, the sharp corners of the arrowhead-like blade could catch on and injure eye tissue surrounding the entry incision. Accordingly, this instrument is also not suitable for goniectomy surgery.
U.S. Pat. Nos. 3,844,272 (Banko I); 3,929,123 (Jamshidi); and 3,007,471 (McClure) disclose surgical instruments which utilize tapered and pointed distal cutting tips in combination with coaxial members such that a sheath can slidably expose or cover the cutting tip. McClure discloses the use of an internal stylet (reference numeral 40) inside the sliding coaxial members. Other patents which show surgical sampling instruments having slidable, coaxial members, internal stylets or both include U.S. Pat. Nos. 4,308,875 (Young); 2,850,007 (Lingley); 3,893,445 (Hofsess); and 4,282,884 (Boebel).
In addition, there are a number of recent patents which are specifically directed to instruments for ophthalmic surgery. These patents include U.S. Pat. Nos. 4,590,935 (Ranalli); 4,577,629 (Martinez); 4,538,611 (Kelman); 4,570,632 (Woods); 4,041,947 (Weiss); and 4,320,761 (Haddad). These patents include discussion of such issues as the importance of irrigation during eye surgery and similar matters which have special relevance for ophthalmic surgery. None of these various instruments is suitable, however, both as a surgical tool and as a device for excising relatively large, undamaged tissue segments.